1. Field of the Invention
The present invention relates to steel strip continuous annealing apparatuses, and more particularly to a steel strip continuous annealing apparatus comprising a heating zone, a soaking zone and cooling zones.
2. Description of the Prior Art
Recently, annealing processes for rendering predetermined processability, deep drawing properties and the like to cold-rolled steel strips have been carried out by continuous annealing apparatuses. These continuous annealing apparatuses are formed into peculiar predetermined configurations depending upon grades of steel, thickness of sheet, temperatures for heating and soaking, cooling conditions and the like.
More specifically, a continuous annealing apparatus for producing black tinplates, for example, has such a function that, in which, a steel strip having a sheet thickness of 0.15 to 0.6 mm and a sheet width of 600 to 1000 mm is soaked at a temperature of 700.degree. to 800.degree. C., thereafter, slowly cooled to about 450.degree. C. from the temperature described above, and further, rapidly cooled to 100.degree. C. to substantially room temperature, where the steel strip is not oxidized, outside the furnace, and consequently, the continuous annealing apparatus comprises a heating, a soaking and a slowly cooling and a rapidly cooling zones. In contrast thereto, a continuous annealing apparatus for producing cold-rolled steel sheets for drawing has such a function that, in which, a steel strip for producing cold-rolled steel sheet for drawing having a sheet thickness of 0.4 to 1.6 mm and a sheet width of 800 to 1500 mm or a steel strip for producing a soft black tinplate having a sheet thickness of 0.15 to 0.6 mm and a sheet width of 600 to 1000 mm is soaked to a temperature of 700.degree. to 850.degree. C., thereafter, rapidly cooled to a temperature of 300.degree. to 500.degree. C. at a cooling rate of approximately 10.degree. to 100.degree. C./sec, subjected to an overaging treatment being held at the temperature of 300.degree. to 500.degree. C. for 1 to 5 min so as to be satisfactorily softened, and then, rapidly cooled, and consequently, the continuous annealing apparatus comprises a heating, a soaking, a rapidly cooling, overaging and a final cooling zones. Furthermore, a continuous annealing apparatus for producing high-strength cold-rolled steel sheet having a mixed structure has such a function that, in which, a steel strip having a sheet thickness and a sheet width similar to those of the cold-rolled steel sheet for drawing as described above is heated to a temperature of 800.degree. to 850.degree. C., caused to partially generate .gamma. phase in a ferrite structure, rapidly cooled at a cooling rate of approximately 10.degree. to 100.degree. C./sec, and turned into a product as it is. Further, a continuous annealing apparatus for producing silicon steel sheets has such a function that, in which, a steel sheet having a sheet thickness of 0.3 to 0.7 mm and a sheet width of 600 to 1000 mm can be heated to a comparatively high temperature of 800.degree. to 1000.degree. C. and soaked, and thereafter, cooled, and consequently, comprises a heating zone, a soaking zone and a cooling zone. As has been described hereinabove, each continuous annealing apparatus is required to have a peculiar heat cycle depending on the material quality of the steel sheet to be annealed and a peculiar configuration depending on the dimensions of the steel sheet, and, it is difficult to treat in one and the same continuous annealing apparatus the black tinplates, cold-rolled steel sheet for drawing, soft black tinplates, high-strength cold-rolled steel sheets, silicon steel sheets or the like, which are different in heat cycle and dimensions.
However, it is uneconomical to set the rate of conveying the steel strip at an excessively small value in a continuous annealing apparatus because the continuous annealing apparatuses each have a production capacity of 20,000 to 40,000 t/mon on the average. Consequently, it is apparently inadvisable for an enterprise having an amount of customers demand insufficient to constantly and sufficiently operate the respective types of continuous annealing apparatuses to possess the abovedescribed continuous annealing apparatuses meeting the conditions required for the types of steel strips. From the viewpoint as described above, necessity has been voiced for making it possible to selectively treat the black tinplates, cold-rolled steel sheets, high-strength cold-rolled steel sheets or silicon steel sheets in a single continuous annealing apparatus.
Furthermore, the continuous annealing apparatus forms a considerably long continuous line, and therefore, in order to maintain a stabilized operating condition in the furnace, it is necessary to render proper tensions to the steel strip in the furnace. For example, in the rapidly cooling zone where the steel strip is rapidly cooled from the soaked condition, cooling irregularities tend to occur in the steel strip because the steel strip is cooled at a high cooling rate, if the tension are expensively high, then irregular shapes of the steel strip and cooling buckling and the like tend to occur, and consequently, the tension of the steel strip in the rapidly cooling zone is required to be set at the level lower than other zones.
However, in the conventional continuous annealing apparatus, the tension acting on the steel strip in the furnace is regulated to be substantially uniform, and hence, the tension of the steel strip only in the rapidly cooling zone cannot be set at a discontinuously low value.
More specifically, in the conventional continuous annealing apparatus, the conditions of tension in the respective zones of the furnace cannot be controlled independently of one another, thus, presenting the problems including the ruptures in the steel strip due to the movement of the steel strip in a zigzag fashion, damages caused to the furnace, fluctuations in width of the steel strip, irregular shapes of the steel strip, local fluctuations in shapes of the steel strip and the like.